Effects of magnesium on the membrane activity and contraction of the circular muscle of rat myometrium during late pregnancy.

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Abstract
Comparative effects of Mg were studied on circular muscle strips of pregnant rat myometrium near term and during delivery. Frequency and amplitude of spontaneous contraction were markedly depressed by adding 0.6 mM Mg to Mg-free Krebs solution in the muscle strips taken in the morning of days 19 and 22 of pregnancy. Frequency decreased, whereas the amplitude was not depressed by 0.6 mM Mg in muscle strips taken during delivery. Plateau potential dominated in the circular muscle taken in the morning on days 19 and 22, and it was depressed by Mg, leaving the evoked spike potential nearly unaffected. Burst of spike potentials dominated in the circular muscle during delivery, and 0.6 mM Mg did not affect the burst discharge. In contrast to the variable effects of 0.6 mM Mg on the polarized muscle strips of different pregnant stages, application of Mg (0.6-3.6 mM) caused commonly a potentiation of tonic contraction of K-contracture generated in muscle strips of any pregnant stage. Prolonged perfusion with Mg-free Krebs solution (several hours) of the muscle strip taken in the morning on day 22 caused a transformation of membrane activity from a plateau dominant type to a spike dominant one. With the same procedure, the depressant effect of Mg on phasic contractions was progressively decreased.
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